The Stirling cycle is a known thermodynamic process wherein an ideal gas in a closed volume is subjected to constant volume heating from a regenerator, allowed to expand isothermally to do work while heat is added from an external source, cooled in the regenerator at a constant volume, and isothermally compressed with heat transfer to an external dump. The Ericsson cycle is similar to the Stirling cycle, except that the ideal gas responds with isobaric regenerative processes instead of isometric ones.
The Rankine cycle is a somewhat different, but still well known, thermodynamic cycle. In this cycle, a liquid working fluid is heated and experiences a phase change into a gas or vapor at high pressure, the vapor is expanded through an engine such as a turbine to produce work, and, in the case of a closed system, the exhaust gas is condensed by heat rejection and the pumped liquid returned to the heater. However, the minimal work advantage of pumping a liquid is more than offset by the poor thermal efficiency of heat rejection during condensation and heat vaporization during expansion, without regenerative processes. Modified Rankine cycle systems employ reheat and staged regenerators to improve efficiency, but the heat addition no longer occurs at the maximum cycle temperature, as is required to approach Carnot efficiency.
The present invention, as well as the ideal Stirling cycle, operates on the principle that useful work and more optimum thermal efficiency can be obtained by partially compressing and partially expanding a working fluid within a closed volume, if at the same time a portion of the working fluid is transferred between a relatively constant hot temperature chamber and a relatively constant cold temperature chamber. These relatively isothermal hot and cold chambers are interconnected by a flow path that includes a regenerator, and are either supplying heat energy as a source reservoir or taking away heat energy as a sink reservoir. Further, provided these isothermal chambers are operating at the maximum cycle and minimum cycle temperatures, and provided that effective regeneration is utilized, then satisfies two of the several conditions that are necessary for approaching Carnot efficiency.
A regenerator, usually filled with a fine mesh material or the like, absorbs and stores thermal energy during that portion of the cycle when the working fluid passes through it moving from the hot chamber to the cold chamber. The regenerator then gives the stored thermal energy back to the working fluid as it moves from the cold chamber back to the hot chamber during a different portion of the cycle. The provision of this working regenerator is required if an actual machine is to more closely approach the unachievable Carnot efficiency of the ideal Stirling cycle, but its inclusion adds performance degradation, and it is sometimes not present in practical prior art Stirling cycle machines.
The "conventional" Stirling cycle, which is the practical implementation of the Stirling cycle in an actual machine, differs from the ideal Stirling cycle in several ways. The most notable difference is the discontinuous motion in the ideal cycle of the compression and expansion devices, e.g., pistons or diaphragms, versus the generally continuous motion in a practical machine. Moreover, the practical gas Stirling cycle suffers performance degradation due to the "dead volumes", i.e., those internal spaces that cannot practically be expanded or compressed. These dead volumes unavoidably occur in the void space of the regenerator (if one is used), in the connecting passageways, in the heat exchange chambers (if used), and in the cylinder or diaphragm clearance spaces and voids.
Additional shortcomings of conventional Stirling cycle machines include: non-isothermal compression and expansion since these would require infinite rates of heat transfer between the working fluid and the media on the outside of the hot and cold chambers; thermodynamically irreversible processes since friction and flow losses cannot be eliminated; and finally, imperfect regeneration since perfect implies infinite heat transfer. To overcome some of these shortcomings and produce practical outputs, many conventional gas Stirling cycle machines operate at extremely high mean pressures, at high source temperatures, at high volume flow rates with low density working fluids such as hydrogen and helium, and with swept volume ratios of 3 to 1 at best. These design solutions, however, lead to material problems, structural difficulties, leakage around piston seals, and a net effect of machines that to date have generally been uneconomical to market.
Accordingly, there is a need for a more efficient and practical heat machine which takes better practical advantage of the benefits of ideal thermodynamic cycles.